1. Field of the Invention
This invention relates to information transmission systems in which information is transmitted between two or more stations in digital form. More particularly, this invention relates to the reduction of errors in digital information transmission systems by utilizing a controlled signal dithering technique.
In the field of information transmission it is common practice to convert information from analog to digital form prior to transmission from a station at a first location and to re-convert the information received at a second location from digital to analog form. In a typical system, the analog-to-digital conversion is accomplished by sampling successive portions of the analog input signal at a rate sufficient to permit conversion in a theoretically errorfree manner under idealized conditions and generating a substantially constant level signal for the duration of each sampling period, the magnitude of the constant level signal during any given period being representative of the magnitude of the analog signal at the instant of sampling. The magnitude of the constant level signal is limited to a relatively small fixed number of possible values over the entire predetermined amplitude range of the analog input signal, a process termed quantizing, and each value is assigned a different amplitude range or quantizing interval so that all signal amplitudes lying within a specific quantizing interval are converted to a constant level signal having the same magnitude. For example, in a seven bit binary system, an analog input signal having amplitudes lying in the range from zero to 1.28 volts may be quantized into different levels each having a range of 0.01 volts so that input signals having amplitudes lying in the zero level range from -0.005 to +0.005 volts are converted to a zero volt level signal; input signals having amplitudes lying in the range from 0.005 to 0.015 volts are converted to a constant level signal having the magnitude of 0.01 volts; signals from 0.015 to 0.025 volts are converted to a constant level signal having a magnitude of 0.02 volts; etc. The voltage magnitudes 0.005, 0.015, 0.025, etc., defining the end points of each range, are termed the transition points or quantization levels. The intervals between the transition points are termed quantization interval. Ideally the quantization intervals are equal in value and define one significant bit (LSB). At the receiving station, the information transmitted in digital form is ordinarily re-converted to analog form which is accomplished in the inverse manner to the manner described above.
Such systems have found wide application, and they are increasingly being used in telephone systems for transmitting speech and other analog information. Such systems are typically designed to operate over a predetermined range of analog input signal frequencies. For example, in a telephone system application, this range is ordinarily in the audible range from about 300 Hz to about 3400 Hz. System response is limited to this range by filtering of the analog input signals prior to the analog-to-digital conversion by means of a band-pass filter having a pass-band characteristic lying in the 300-3400 Hz range and by filtering the reconverted analog signal with a post-sampling filter having similar pass-band characteristics.
Such systems, however, suffer from the disadvantage of susceptibility to random disturbing signals upstream of the analog digital converter (ADC) and lying in the frequency response range of the system, which signals are conveniently termed noise signals, as opposed to information signals whose information content is to be transmitted to the receiving station. In the presence of noise signals, the information content desired to be transmitted and received can be masked and erroneously manifested at the receiving end of the system. Ideally, under idle channel conditions, i.e., when no information is present on the input side of the system, the output of the ADC should have a constant zero level value. In practice, however, in a typical ADC, the zero level drifts. Thus, a random or spurious disturbing signal having an extremely small amplitude can cause the ADC to generate an output signal quantizing a value higher or lower than zero, if the zero value level has drifted close to a transition point. This erroneous output signal is then reproduced as an erroneous analog signal downstream at the digital-to-analog converter (DAC).
In systems using a multi-channel input which is sequentially coupled to the ADC, i.e., a multiplexed multichannel system, noise in the form of crosstalk from a nearby channel is typically present. Since the crosstalk noise signal has the spectral content of speech and thus lies within the frequency response range of the system, crosstalk signals of even extremely small amplitude can pass through the system band-pass filter and alter the magnitude of the sampled analog information input signal to a value lying within the next quantizing interval, particularly when the input signal alone is very close to a transition point. As a result, the ADC generates an erroneous output signal which is re-converted to analog form by the DAC. Since the spectrum of this signal is fundamentally that of speech any such noise cannot be filtered out by the postsampling filter downstream of the DAC.